1. Field of Application
The present invention relates to a sensor apparatus in which a plurality of detection blocks produce respective signals in accordance with a dynamic quantity that acts from the exterior, with a differential output signal being obtained from the signals of the detection blocks, for thereby canceling deviations due to external disturbing effects and detecting only the required variations in the dynamic quantity.
2. Description of Related Art
For example in the case of an electrostatic capacitance type of microminiature angular velocity sensor or accelerometer (referred to in the following as a micro-angular velocity sensor and a micro-accelerometer respectively), which must detect minute variations in electrostatic capacitance, large changes can occur in the zero-point of an output detection signal from the sensor if variations occur in electrostatic capacitance as a result of causes other than changes in acceleration or in angular velocity.
Such a sensor generally is based on a sensor chip, with a sensor element being mounted on one face of the sensor chip, and changes in capacitance between comb electrodes of the sensor element being detected to obtain an output signal. There is a specific problem at present concerning bending deformation of the sensor chip of this type of sensor. This deformation may be caused by thermal distortion, etc., of a package that contains the sensor chip, or of a printed circuit board on which such a package is mounted. This is a problem in that such deformation of the sensor chip can produce corresponding deformation of the sensor element that is mounted on the sensor chip, and thereby alter the spacings between the comb electrodes of the sensor element, or areas of overlap between respective comb electrodes. Variations in these spacings or overlap areas are detected (as electrical signals resulting from changes in electrostatic capacitance), to detect changes in a dynamic quantity such as acceleration. Thus, the zero point of such detection will deviate and the detection accuracy will thereby be lowered, when such deformation of the sensor chip is transmitted to the sensor element.
In the case of a known type of sensor having a multi-layer chip configuration, as described for example in Japanese patent first publication No. 2006-98168 (referred to in the following as reference document 1), the effects of thermal distortion are reduced by disposing a material such as a soft adhesive film between the chips. However such measures are not sufficient to prevent the above-described variations in the zero point.
In the element structure of a micro-angular velocity sensor or a micro-accelerometer, the element is basically divided into a plurality of detection blocks (e.g., each having a pair of opposing intermeshed comb electrodes). A differential circuit obtains a differential output signal from respective detected capacitance amounts that are obtained by the detection blocks. Extraneous variations in electrostatic capacitance due to certain specific causes can thereby be cancelled, with only the required changes in electrostatic capacitance (i.e., caused by changes in a dynamic quantity such as acceleration or angular velocity) being detected.
In the case of a micro-angular velocity sensor, the difference between the outputs from respective differential circuits is obtained, to thereby cancel variations in the electrostatic capacitance that are caused by externally applied shock. In the case of a micro-accelerometer, the difference between the outputs from two detection blocks is obtained. The respective set values of electrostatic capacitance of these detection blocks may deviate, or the zero point may deviate substantially for each of the detection blocks, due to causes such as manufacturing variations in vibrator elements, etc. By using the difference between the respective outputs from these two detection blocks, the effects of deviations in the set values and in the zero point can be cancelled.
Such an arrangement also enables cancellation of the effects of specific types of chip deformation on electrostatic capacitance in a sensor element, i.e., types of deformation as illustrated in FIGS. 9A and 9B. In the following, an axis which connects the centers of opposing sides of a sensor chip or a sensor element will be referred to as a central axis of the chip or element. The type of deformation shown in FIGS. 9A, 9B can be caused by thermal distortion. In the example of FIG. 9A, in a miniaturized sensor apparatus such as an micro-accelerometer, a sensor chip 101 has a sensor element 102 mounted on its upper face, and the sensor chip 101 has bow-shape deformation with respect to one of its central axes. In the example of FIG. 9B there is similar bow-shape deformation about the other central axis of the sensor chip 101. Changes in values of electrostatic capacitance (obtained from the sensor element 102) which result from this type of deformation can be cancelled by obtaining a differential output, as described above, e.g., by using the difference between respective output signals from two detection blocks in the sensor element 102.
However it is also necessary to consider the form of deformation illustrated in FIG. 10, in which bending forces act at the corners of the sensor chip 101. This results in bending deformation of the chip by forces acting on diagonally opposing corners of the chip. Cancellation of the changes in electrostatic capacitance which result from this type of deformation cannot be achieved to a sufficient extent, by simply obtaining a differential output. Hence, deviation of the zero point will occur. This represents a serious problem with respect to accurate operation of a micro-angular velocity sensor or micro-accelerometer.